1. Field of Invention
This invention pertains generally to anaerobic digesters and, more particularly, to a system and process that significantly extends the efficiency, control, and applicability of anaerobic digesters to all of the many and variously different liquefied bio-waste products over a wide variety of conditions and concentrations.
2. Related Art
Energy costs have always been a concern in wastewater treatment facilities, with larger plants being required to handle hydraulic loads of millions of gallons per day.
Such plants have generally involved relatively large structures, with different parts of them being designed by specialists in the different functions which they perform. As a result, the costs of design and operation have been higher than they might have been if the design process were better integrated.
U.S. Pat. No. 5,185,079 describes a sequenced batch process which is widely used in the industry at this time. However, waste streams are almost always continuous and often vary dramatically in flow rates, and batch processes of that type cannot be operated in a continuous mode without losing large quantities of solids to allow room for influent liquor, or supernatant. They must be shut down to allow for settling of the solids, otherwise the supernatant that is drawn off and returned to the plant influent is loaded with solids that must again be separated.
U.S. Pat. No. 5,540,839 discloses another cyclical degradation process, utilizing a combination of mesophilic and thermophilic steps, which is said to degrade organic matter completely to gaseous products. It does not, however, teach the use of such a process in a continuous flow mode.
U.S. Pat. No. 5,630,942 discloses a two phase anaerobic digestion process utilizing thermophilic fixed growth bacteria in which different phases of the digestion process are carried out in different tanks.
Heretofore, a major drawback to anaerobic treatment processes has been the manner in which excess liquid is controlled and eliminated. That has commonly been done by a technique commonly known as supernatant wasting in which putrid thin liquor is drained off and pumped back to the plant inflow in order to maintain the minimum concentration of volatile solids required for anaerobic colonization and the degradation of bio-solids to take place. That is an energy consuming process which also increases the volumetric capacity requirements of the system and requires the operation of a plug flow procedure in order to allow settling of the solids.
Processes for treating domestic wastewater, liquefied bio-waste, and commercial and industrial liquid waste have historically used two distinct classes or systems of bacteria to reduce the bio-solids contained therein to gases and to more biologically stable organic and inorganic matter. While one ideally might want to completely destroy the organic fraction of this mixture, the energy intensity and extended reaction times of a system for doing so would become physically impractical and economically prohibitive. The two bacterial systems in use today are the aerobic and anaerobic systems.
Aerobic processes require the mixing of air or pure oxygen into the liquor being treated so that aerobic bacteria known as aerobes grow, attack, and biochemically reduce the solids. Aerobic processes are relatively easy to devise, and there are many such systems in use throughout the world.
A desire for higher and higher quality effluents has contributed to the expansion and proliferation of aerobic processes. However, there are a number of disadvantages to aerobic processes. They are generally open processes that have odor problems, they tend to require multiple large tanks or ponds that have big footprints and require considerable space, they consume large quantities of energy in the form of electrical power, and produce large quantities of greenhouse gases. Approximately, 60% to 70% of the energy required in modern domestic wastewater treatment plants is directly attributed to aerobic processes.
Conversely, anaerobic processes can be net energy producers. They operate in closed tanks or vessels devoid of oxygen, at an elevated temperature. Although they are sometimes more difficult to control, they produce a raw or “oil” gas that generally contains approximately 64% methane (natural gas), 34% carbon dioxide, and 2% hydrogen sulfide. Mixed with these gases as removed from a digester are quantities of water vapor, which vary from about 5% to 10% at mesophilic temperatures (about 35° C.) to about 30% to 35% at thermophilic temperatures (about 55° C.), rendering the use of vacuum pumps and blowers totally impractical. Even with the best water traps and cooling systems, maintaining the kinds of pumps and blowers used to increase gas line pressure for mixing and burning is a continual problem.
Another problem with modern digesters is that they cannot be operated at a negative dome pressure. The roofs of such digesters, especially floating types, are not structurally designed for such loading, there are too many places where air can leak into the dome space and produce a dangerous explosive mixture.
The limiting factor that has prevented all wastewater feedstock from being treated anaerobicly is the high ratio of water to bio-solids (volatile solids) contained in the feedstock. Domestic wastewater typically contains as little as 0.01% volatile solids, whereas it is generally difficult to maintain anaerobic action below a minimum threshold of about 3% to 5% volatile solids. Therefore, anaerobic digestion is commonly limited to a relatively small part of the influent that either settles readily or floats to the top of large primary and secondary sedimentation tanks, leaving a very large portion of the influent to be processed by aerobic activated sludge processes.
The energy produced by anaerobic systems in the form of methane gas is a direct function of the quantity of volatile solids or biomass reduced in the process. Therefore, the net positive energy generated is generally limited severely by the ratio of water to volatile solids in the digester influent, irrespective of several chemical-thermal-mechanical factors that determine digester efficiency.
Also, depending upon the feedstock, there has generally been an operating point at which it becomes more efficient to transfer a portion of the treated influent to aerobic processing. This limitation can be overcome to some extent by the addition of external bio-solids (e.g., food, animal or agricultural solids, grass clippings, tree trimmings, cardboard, and other bio-waste products) to the anaerobic influent.
The inside of an anaerobic digester is a dirty, foul and dangerous place to work. Ideally, one would like to build a digester and never have to look inside for its expected life span of some thirty years. “Pancake” digesters, i.e. large diameter, low aspect ratio structures that are common in the U.S. but frowned upon in Europe, must be cleaned every several years. It is not uncommon to find several feet of “muck” at the bottom of a digester tank with vortices opening down to the pump-out inlets and to find as much as a foot of matted scum held together by hair, string, rags and the like floating on the surface of the liquor. Muck and scum both greatly decrease the effective size of the digester and, therefore, limit its efficiency.
Digesters of the type shown in U.S. Pat. No. 6,291,232 significantly reduce or eliminate the build-up of muck at the bottom of the digester tank. However, scum is still a real nuisance and hindrance to the digestive process. Cooking oils and greases, for example, form a stiff molasses type of scum that restrains gas bubbles and results in foaming that carries the oily bubbles up to the outlet and instrumentation ports, causing false instrument readings and clogging gas vents. The problem gets even worse as digester concentrations are made higher and more garbage, slaughterhouse wastes and solid wastes are brought into the digester.